It's like

system($^X, -le => '$|=1; { print ++$i; sleep 1; redo; }')
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but it doesn't wait for the child to end before continuing, and the child's STDOUT ends up in $fr_chld instead of being sent to the parent's STDOUT.

As for the command itself, my demo program runs Perl ($^X) to execute an infinite loop ({ ...; redo; }) which outputs a number every second (print ++$i; sleep 1;).